1. Field of the Invention
The present invention relates to a beam homogenizer for homogenizing a beam spot on a surface to be irradiated in a certain region. The present invention also relates to a laser irradiation apparatus for irradiating the surface to be irradiated with the beam spot. Furthermore, the present invention also relates to a method for manufacturing a semiconductor device using a crystalline semiconductor film formed with the laser irradiation apparatus.
2. Description of the Related Art
In recent years, there has been a technique widely studied for crystallizing or enhancing a crystallinity of an amorphous semiconductor film or a crystalline semiconductor film (a semiconductor film having a crystallinity such as poly-crystal or micro-crystal, which is not single-crystal), that is to say, a semiconductor film which is not single-crystal (referred to as a non-single crystalline semiconductor film) formed over an insulating surface such as a glass substrate with laser annealing performed thereto. A silicon film is often used as the semiconductor film.
In comparison with a quartz substrate that has been used conventionally, the glass substrate has advantages that it is inexpensive and superior in workability, and that it can be processed easily into a large sized substrate. This is the reason why the study has been extensively conducted. The laser is preferably used for crystallization because the glass substrate has a low melting point. The laser can give high energy only to the non-single crystal semiconductor film without changing the temperature of the substrate too much.
The crystalline silicon film formed with the laser annealing has a high mobility. Therefore, a thin film transistor (TFT) formed with this crystalline silicon film is used extensively. For example, the crystalline silicon film is extensively used in a monolithic liquid crystal electro-optical device and the like in which TFT for a pixel and TFT for a driver circuit are formed on one glass substrate. The crystalline silicon film is referred to as a poly-crystalline silicon film or a poly-crystalline semiconductor film because the crystalline silicon film is formed of a number of crystal grains.
In addition, it is possible to shape a laser beam oscillated from a pulsed laser oscillator having high output such as an excimer laser into a square spot with several cm on a side or into a linear spot with 10 cm or more in length (for example, Japanese Patent Publication No. 9-234579). Then the beam spot is moved relative to the surface to be irradiated to perform the laser annealing. Since such a method can enhance productivity and is superior industrially, it is preferably employed.
In particular, when the linear beam spot is employed, unlike a punctate beam spot requiring to be scanned from front to back and from side to side, the linear beam spot can provide high productivity since a large-sized surface can be irradiated by scanning the linear beam spot only in a direction perpendicular to the direction of its major axis. It is noted that the linear beam spot here means a rectangular beam spot having a high aspect ratio. The beam spot is scanned in the direction perpendicular to the direction of the major axis of the linear beam spot because it is the most effective scanning direction. Because of such high productivity, at present, the laser annealing process is mainly employing the linear beam spot obtained by shaping a pulsed excimer laser through an appropriate optical system.
FIGS. 6A and 6B show an example of the optical system for shaping a cross-section of a beam spot into linear on the surface to be irradiated. The optical system shown in FIGS. 6A and 6B is an extremely general optical system. The optical system not only shapes the cross-section of the beam spot into linear but also homogenizes the energy of the beam spot on the surface to be irradiated simultaneously. Generally, the optical system for homogenizing the energy of the beam is referred to as a beam homogenizer. The optical system shown in FIGS. 6A and 6B is also a beam homogenizer.
First, a side view of FIG. 6A is explained. A beam spot of a laser beam oscillated from a laser oscillator 1201 is divided in one direction through cylindrical lens arrays 1202a and 1202b. The direction is referred to as a vertical direction. When a mirror is inserted in the optical system, a beam spot in the vertical direction is bent to the direction bent by the mirror. The laser beam is divided into four beams in this structure. These divided beam spots are combined into one beam spot with a cylindrical lens 1204 once. After the beam spots separated again are reflected on a mirror 1207, the beam spots are converged into one beam spot again with a doublet cylindrical lens 1208 on a surface to be irradiated 1209. A doublet cylindrical lens is a set of lenses consisting of two cylindrical lenses. The configuration of FIGS. 6A and 6B homogenizes the energy distribution of the beam spot shaped into linear in the vertical direction and determines the length thereof in the vertical direction.
Next, a top view of FIG. 6B is explained. The beam spot of a laser beam oscillated from the laser oscillator 1201 is divided in a direction perpendicular to the vertical direction through a cylindrical lens array 1203. The direction perpendicular to the vertical direction is referred to as a horizontal direction. When a mirror is inserted in the optical system, a beam spot in the horizontal direction is bent to the direction bent by the mirror. The beam spot is divided into seven beam spots in this structure. After that, the beam spots divided into seven beam spots are combined into one beam spot on the surface to be irradiated 1209 with a cylindrical lens 1205. A dotted line shows correct optical path and correct positions of the lens and surface to be irradiated in the case not disposing the mirror 1207. The configuration of FIGS. 6A and 6B homogenizes the energy distribution of the beam spot shaped into linear in the horizontal direction and determines the length thereof in the horizontal direction.
As described above, the cylindrical lens arrays 1202a, 1202b, and 1203 are the lenses for dividing the beam spot of the laser beam. The number of the divided beam spots determine the homogeneity of the energy distribution of the obtained linear beam spot.
Each of the lenses is made of quartz in order to correspond with the XeCl excimer laser. In addition, the lenses have coated surfaces thereon so that the laser beam emitted from the excimer laser transmits through the lenses very much. This makes transmittance of the excimer laser beam become 99% or more per one lens.
The linear beam spot processed through the above lenses is irradiated as being overlapped in such a way that the linear beam spot is displaced gradually in the direction of the minor axis of the linear beam spot. With such irradiation performance, the laser annealing can be conducted to the whole surface of the non-single crystal silicon film, for example, so as to crystallize it or to enhance its crystallinity.
Next, a typical method for manufacturing a semiconductor film, which becomes an object to be irradiated by the laser beam, is shown. Initially, a glass substrate having a thickness of 0.7 mm and a length of 5 inch on a side is used. A SiO2 film (a silicon oxide film) is formed over the substrate in 200 nm thick with a plasma-CVD apparatus, and an amorphous silicon film (hereinafter referred to as a-Si film) is formed over a surface of the SiO2 film in 50 nm thick. When the substrate is exposed to the atmosphere of nitrogen at a temperature of 500° C. for one hour, hydrogen concentration in the film is decreased. The resistivity of the film is considerably increased against the laser beam.
A XeCl excimer laser (wavelength 308 nm, pulse width 30 ns) is used as the laser oscillator. A spot size of the laser beam is 15 mm×35 mm at the laser beam window (both are width at half maximum). The laser beam window is defined as a plane perpendicular to the traveling direction of the laser beam just after the laser beam is emitted from the laser oscillator.
The laser beam emitted from the excimer laser usually has a rectangular shape, and when it is expressed with an aspect ratio, the rectangular beam has an aspect ratio ranging from 1 to 5 approximately. The laser beam has Gaussian energy distribution in which the intensity of the laser beam becomes higher toward the center thereof. The beam spot of the laser beam is changed into a linear beam spot having homogeneous energy distribution and having a size of 300 mm×0.4 mm through the optical system shown in FIGS. 6A and 6B.
When the laser beam is irradiated to the semiconductor film, about 1/10 of the minor width (width at half maximum) of the linear beam spot is the most appropriate pitch for overlapping the laser beam. The homogeneity of the crystallinity in the semiconductor film can be improved. In the above example, since the minor width is 0.4 mm, the laser beam is irradiated under the condition of the excimer laser in which the pulse frequency is set to 300 Hz, and the scanning speed is set to 10 mm/s. On this occasion, the energy density of the laser beam on the surface to be irradiated is set to 450 mJ/cm2. The method described above is a very general method for crystallizing the semiconductor film with the linear laser beam.
The cylindrical lens requires to be manufactured with high accuracy.
The cylindrical lens array is the lens with cylindrical lenses arranged in a direction of its curvature. Here, the direction of the curvature is defined as a direction perpendicular to a generating line of a cylindrical surface of the cylindrical lens. The cylindrical lens array always has a joint between the cylindrical lenses constituting the cylindrical lens array. Since the joint does not have a curvature as the cylindrical lens, the laser beam being incident into the joint is transmitted without being influenced by the cylindrical lens. The laser beam reaching the surface to be irradiated without being influenced by the cylindrical lens may cause inhomogeneity of the energy distribution of the rectangular beam spot on the irradiated surface.
In addition, all the cylindrical lenses constituting the cylindrical lens array must be manufactured with the same accuracy. When the cylindrical lens has a different curvature, the laser beams divided by the cylindrical lens array are not overlapped on the same position in the surface to be irradiated even with a converging lens. In other words, the region where the energy is attenuated in the rectangular beam spot on the irradiated surface increases. This causes the lowering of the energy usability.
The cause of the inhomogeneous energy distribution of the beam spot on the irradiated surface lies in the structural problem and the manufacturing accuracy of the cylindrical lens array constituting the optical system. More specifically, one of the reasons of the inhomogeneous energy distribution is that all the laser beams divided by the cylindrical lens array are not overlapped on the same position.
Furthermore, when the semiconductor film is irradiated and scanned with the rectangular beam spot having inhomogeneous energy distribution in the direction of its major axis on the surface to be irradiated, the crystallinity of the semiconductor film becomes inhomogeneous in a reflection of the inhomogeneous energy distribution. The inhomogeneity of the crystallinity is synchronized with the inhomogeneity of the characteristic of the semiconductor film such as the mobility. For example, the inhomogeneous crystallinity appears as a variation of an electric characteristic of the TFT comprising the semiconductor film, and displays light and shade pattern on a panel comprising the TFT.
The present invention is made in view of the above problem. The present invention provides a beam homogenizer being able to form a rectangular beam spot having homogeneous energy distribution in the direction of its major axis on the irradiated surface without using the optical lens that is necessary to be manufactured with high accuracy. In addition, the present invention provides a laser irradiation apparatus being able to irradiate a laser beam having a beam spot with homogeneous energy distribution in the direction of its major axis. Furthermore, the present invention provides a method for manufacturing a semiconductor device, being able to enhance the crystallinity of a semiconductor film and to manufacture a TFT with high operating characteristic.